The invention described herein may be manufactured, used, imported, sold, and licensed by or for the Government of the United States of America without the payment to me of any royalty thereon.
The present invention relates generally to the field of lithium batteries, and more particularly to providing reducing agents that prevent gas formation and electrolyte polymerization in lithium organic manganese dioxide electrochemical cells.
Portable batteries with increased energy and power densities are required as the use of portable electronic equipment by the military and civilians continues to rapidly increase. Additionally, these batteries must operate safely in all environments. Presently most military applications use the primary lithium/sulfur dioxide electrochemical system introduced to the field in the early 1980""s, which provide significant increases in capacity, rate and low temperature operation capability. Although the lithium/sulfur dioxide system has served its purpose well, today""s needs for portable electric power require that such primary batteries contain more energy and more power, as well as matching or surpassing the safety of current batteries. The lithium/manganese dioxide (Li/MnO2) electrochemical system may become the future primary battery in numerous military and civilian applications.
In fabricating lithium organic electrolyte manganese dioxide cells, the pouch cell is a significant manufacturing technique that removes the constraints of metal cylindrical cans from battery design by housing the active battery materials in a flat, light weight pouch that can take on any shape or size. However, the pouch cell imposes a number of difficulties on battery chemistry. For example, properties like electrolyte vapor pressure and electrode expansion on discharge became more important in the pouch cell because the metal can no longer contains pressure within the cell. After assembly, the lithium organic electrolyte manganese dioxide tends to become gassy and can vent unhealthy internal components to the user. When these pouch cells are discharged immediately after fabrication, the dangerous gases no longer seem to form. However, even a pre-discharged lithium organic electrolyte manganese dioxide pouch cell can exhibit gassing. Also, the cells show signs of electrolyte polymerization, which can occur in a few days or after the pouch cells are stored.
Several techniques have evolved to overcome the problems of gas formation and electrolyte polymerization in lithium organic electrolyte manganese dioxide (Li/MnO2) pouch cell production. One method for eliminating gas formation in the cell is to discharge the pouch cell immediately after assembly, but this technique suffers from the disadvantage of removing about ten percent of the cell""s total capacity. It has also been found that residual water within cell components is responsible for electrolyte decomposition producing gasses and the formation of alcohols. When cyclic carbonates, such as propylene carbonate, are used in fabricating Li/MnO2 cells, the alcohol formed in the decomposition of the electrolyte is a dialcohol that can polymerize the electrolyte. This electrolyte polymerization reduces cell performance and can make the cell unusable. Though problems such as electrolyte side reaction were first observed in connection with Li/MnO2 pouch cells, these shortcomings and limitations are not limited to pouch cells.
The current manufacturing process of cell discharge shortly after cell completion is ineffective, because even though discharge usually eliminates the gassing problem, polymerization is still often observed after cell storage. Moreover, when Li/MnO2 cells are discharged immediately after manufacturing, that discharge removes about ten percent of the capacity of the cell and lowers the open circuit voltage of the system from 3.6 to under 3.2 volts. Thus there has been a long-felt need to address the degradation of the electrolyte in Li/MnO2 and other lithium cells and focus on the chemical mechanism causing electrolyte decomposition and resulting in gas formation and electrolyte polymerization in Li/MnO2 and other lithium cells, without engaging in costly post-manufacture discharge. The reducing agents of the present invention eliminate the disadvantages of gassing and polymerization due to electrolyte decomposition and thus, increase production efficiency for lithium organic electrolyte manganese dioxide cells.
In order to overcome the long-felt problems of gas formation and polymerization in Li/MnO2 and lithium cells, the present invention provides a reducing agent for polymerized lithium organic electrolyte electrochemical systems. The present invention comprises adding a phosphorous or arsenic reducing agent to the Li/MnO2 and other lithium battery electrolyte that generates ether and certain acids, which prevents both gas formation and electrolyte polymerization caused by the formation of dialcohol.
It is an object of the present invention to provide a phosphorous reducing agent to eliminate gas formation and polymerization of Li/MnO2 and other lithium cells caused by the formation of dialcohol.
Another object of the present invention is to provide a phosphorous reducing agent to the Li/MnO2 battery electrolyte causing the phosphorous compound and alcohol to react and produce ether and orthophosphorous acid, which prevents gas formation and the polymerization of Li/MnO2 cells caused by the formation of dialcohol.
It is still another object of the present invention is to provide an arsenic reducing agent to the Li/MnO2 and other lithium battery electrolyte causing the arsenic compound and alcohol to react and produce ether and orthoarsenic acid, which prevents gas formation and the polymerization of L/MnO2 cells caused by the formation of dialcohol.
These and other objects are advantageously accomplished with the phosphoric reducing agent of the present invention, comprising a reducing agent with the general formula P(OR)3 where Rxe2x95x90CH3CH2, CH3, (CH3)2CH) being added to a Li/MnO2 battery electrolyte, causing a reaction between the reducing agent and alcohol that produces ether and orthophosphorous acid. The ether and orthophosphorous acid produced by this reaction prevent gas formation and the polymerization caused by the formation of dialcohol in the Li/MnO2 cells. The preferred embodiment of the present invention is a phosphoric acid tri-ester. Further, other phosphorous compounds cause the necessary reaction between the phosphate compound and alcohol to produce ether and orthophosphorous acid compounds. Other useful reducing compounds that provide the same reactions and similar effects include arsenic (As) and halogenated hydrocarbons such as fluorine, chlorine, bromine and iodine, as well as phosphoric acid tri-ester, phosphorous tri-fluoride, phosphorous tri-chloride, phosphorous tri-bromide, phosphorous tri-iodide, arsenic acid tri-ester, arsenic tri-fluoride, arsenic tri-chloride, arsenic tri-bromide and arsenic tri-iodide.